This invention relates generally to systems which include an electromagnetic flowmeter whose electromagnet is excited by a pulsatory current having a predetermined drive frequency to produce an analog signal indicative of flow rate, and more particularly to a system in which this analog signal is converted into a digital output representing instantaneous flow values that may be processed by an intelligent digital device such as a microprocessor.
In a conventional electromagnetic flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the fluid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the fluid and hence to its average volumetric rate, is then amplified and processed to yield an output signal for actuating a recorder or indicator, or for carrying out various process control operations.
The magnetic field may be either direct or alternating in nature; for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible; for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow-induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublesome are stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but elimination of its in-phase component has not been successful. Existing A-C operated electromagnetic flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltages both with respect to phase and magnitude. Hence it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change in the flux field d.phi./dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But d-c operation to create a steady state field is not acceptable; for galvanic potentials are produced and polarization is encountered, as previously explained. In order, therefore, to obtain the positive benefits of a steady state field without the drawbacks which accompany continuous d-c operation, the U.S. Pat. No. 3,783,687 to Mannherz et al. (hereinafter referred to as the Mannherz patent) discloses an excitation arrangement in which the steady state flux field is periodically reversed or interrupted. The entire disclosure of this patent is incorporated herein by reference.
In the Mannherz patent, in order to avoid the spurious voltages which result from stray couplings without, however, causing polarization of the electrodes, the electromagnet is energized by a low-frequency square wave. This wave is produced by applying the output voltage of an unfiltered full-wave rectifier to the electromagnet and periodically interrupting this voltage at a low-frequency rate by means of an electronic switch.
The magnetic field produced by the square wave is disrupted by switching transients. In a flowmeter of the Mannherz type, the flow-induced signal derived from the electrodes is measured during a sampled portion of each "on" and each "off" condition of the magnetic field in the course of an excitation cycle to discriminate against the transients. The converter to which the signal from the electrodes is applied includes a synchronous demodulator which is gated synchronously to yield an output signal only when the magnetic flux achieves a steady state condition. Successive differences in this signal are taken as representative of flow rate.
When the Mannherz electromagnetic flowmeter is a component in an industrial process control loop, it becomes necessary to compare its flow rate indication in in electronic controller with a set point to provide an output which depends on the deviation of the flow rate from the set point, which output acts to govern the operation of a final control element, such as a valve, to change the flow rate in a direction and to an extent determined by the set point. For this purpose, it is useful to employ a digital computer as the electronic controller; for the computer is capable of conditioning the flowmeter signal and of serving in a multiplex arrangement in conjunction with a plurality of process control loops.
A digital computer is an instrument adapted to carry out arithmetic or logic operations on digital data entered into its input to yield numerical results or decisions. All digital computers, whether in large-scale general-purpose form or in microcomputer form, are essentially composed of a central processing unit, a memory system and input-output devices.
The task assigned to a central processing unit is to receive and to store digital data for later processing, to perform arithmetic or logic operations on this data in accordance with previously-stored instructions, and to deliver the results in the form of a digital output signal.
The central processing unit is that component of the computer which controls the interpretation and execution of instructions. A microprocessor is the central processing unit of a computer with its associated circuitry that is scaled down by integrated-circuit techniques to fit on one or more silicon chips containing thousands of transistors, resistors or other electronic circuit elements. By combining a microprocessor with other integrated circuit chips that provide timing, random access memory, interfaces for input and output signals and other ancillary functions, one can thereby assemble all of the necessary components of a microcomputer whose master component is the microprocessor.
In order to process the analog signal yielded by an electromagnetic flowmeter whose electromagnet is excited by a low frequency pulsatory current, such as a flowmeter of the type disclosed in the Mannherz patent, one must first convert this signal into a digital output representing instantaneous flow values. By digital processing, one is also able to eliminate or minimize the effect of noise components on the flow induced signal.